The invention disclosed herein relates to an improved apparatus for automatically regulating the pumping of an oil well based upon the detected liquid level in the well.
Unfortunately, few oil wells in the United States are self flowing and therefore most wells must be pumped. Usually, wells have a pump near the bottom of the borehole that is activated by a string of sucker-rods extending down through the borehole to the pump. The sucker-rods are attached to a polish-rod at the surface. The polish-rod extends through a stuffing box and is attached to the mechanical unit that produces the necessary reciprocating motion to actuate the sucker-rods and the pump. Typically, the polish-rod is attached to a walking beam pivotally mounted to a post. A counter balancing weight may be directly or indirectly attached to the opposite end of the beam. As the beam is rocked by the action of a motor, the sucker-rods are raised and lowered.
In typical operation, the oil in the borehole is pumped out. Then pumping is discontinued and oil and sometimes salt water is allowed to seep into the borehole from the surrounding oil-bearing formation. Build up of liquids in the borehole produces a back pressure which impedes the inflow from the formation. Thus, productivity is reduced if the oil in the borehole is not timely removed after accumulating. On the other hand, it is not desirable to operate the pump after the oil level has fallen below the pump inlet. To do so causes physical damage and wearing of the pump and unnecessary wear on the entire system.
Past and present practice in oil well pumping involves manually setting electric timers to control the pumping based upon an operator's estimation of the time required to extract the down-hole fluid to a point where no more fluid can be removed. This is known as "pumping off" the well. The disadvantages of this method are excessive wear to down-hole components due to excessive pumping, unnecessary man-hours needed to operate the wells and lower fluid production reducing income due to under pumping.
Automatic liquid level monitoring has long been proposed and the literature includes teachings of proposed schemes for automatic control. See, for example, U.S. Pat. No. 4,392,782. As emphasized in the prior art, there are two basic problems to overcome: transferring or generating electrical power down-hole for the level sensor and transferring the liquid level information back up the hole. U.S. Pat. No. 4,318,298 discloses an apparatus that is entirely at the wellhead to avoid both of these problems by using an acoustical device to sense the depth of the liquid within the well. A drawback of the system is that it is affected by foam within the well. Also the mechanical apparatus at the wellhead is complicated. U.S. Pat. Nos. 3,437,992 and 4,570,718 disclose approaches to solving the problem of supplying power to a down-hole sensor by using the mechanical motion of the casing or the changes in pressure during pumping to drive a down-hole generator. The '992 patent teaches transferring the information up hole by direct current pulses applied across the casing and tubing. The '718 patent teaches transferring the information up hole acoustically.
The preferred liquid level detectors used in the apparatus according to the applicants' invention sense change in capacitance between two plates. It is a characteristic of many oil wells that the borehole not only fills with oil but also with salt water. The oil having the lower density floats on the top of the salt water. The presence of salt water creates a number of problems. Salt water will short out the plates of the capacitive detector unless they are electrically insulated. Salt water will short the tubing to the casing making them useless for transferring power down-hole or information up hole. Finally, salt water between the plates of the capacitive detector will change the reading by the capacitive detector much more dramatically than will oil. The relative capacitance readings of air, oil and salt water are 1, 2-6, and 70 respectively.